The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
The physical testing of materials and/or components by taking a test specimen and applying tension and/or compressive loads and/or displacements using an actuator is well known. Commonly, the tension and compression loads are applied to the test specimen in an alternating manner at a selected frequency, or through a range of frequencies at constant displacement or amplitude. In harmonic motion, such as present in this form of testing, the acceleration of moving components of the actuator, the specimen grips, etc. are proportional to the amount of displacement multiplied by the square of the frequency. Therefore, even if the amplitude is small (e.g. 0.06 mm), the acceleration can be very large at higher frequencies (e.g. 700-1000 Hertz).
Consequently, the force, which is proportional to the mass of the moving components times the acceleration, is also increasing by the square of the frequency, as the frequency increases. Moreover, this force must be reacted by the structure of the test system, which will cause excitation of modes in the test system.
A common test system construction includes a base with upstanding columns that support a crosshead over the base. A first specimen grip is coupled to the crosshead through the actuator, while a second specimen grip is coupled to the base through a force transducer; however the location of the actuator and force transducer can be reversed.
Due to the large dynamic forces, one mode of vibration is that the columns may stretch and compress allowing the crosshead to move up and down slightly. However, another mode of vibration that is also proving to be detrimental to testing is a box mode excited in the box-like construction of the crosshead, base and columns. These modes are detrimental because these modes causes the force transducer to move up and down, which induces an inertial error in its corresponding output signal. Many other vibration modes of this type can be conceptualized and these detrimental modes can be referred to as structural vibration modes.